1. Field of the Invention
The present invention relates to a diaphragm for a diaphragm pump.
2. Description of Related Art
FIG. 1 shows a diaphragm pump that is used in reverse osmosis water purification systems. The diaphragm pump 10 contains a piston 12 that is reciprocated by a wobble plate assembly 14 within a pumping chamber 16 of the pump 10. The wobble plate is typically driven by an electric motor (not shown). The reciprocating movement of the piston 12 pumps water out of the pumping chamber 16 and through an outlet port of the pump 10. There are typically three separate pistons that move in synchronization to continuously pump water from the unit. The pistons 12 are attached to a diaphragm 18 which is captured by the outer shells 20 of the pump housing. The outer edge of the diaphragm 18 has a lip 22 which seals the pump housing.
The unsupported portion of diaphragm 18 must be strong enough to withstand the hydraulic pressure generated within the pumping chamber. At the same time, the diaphragm must be capable of flexing repeatedly without deteriorating.
Diaphragms have been produced utilizing thermosetting elastomeric materials cured around a reinforcement fiber mat. It was found that as the diaphragm flexed, the individual strands in the fabric weave would move against each other, eventually wearing to the point of failure. Thermoplastic materials were traditionally too stiff to flex sufficiently to be considered for diaphragms. However, by mixing a base plastic polymer with a certain amount of elastomeric filler, a "thermoplastic elastomer" can be generated with favorable flexing characteristics. Montasanto Co. has sold a thermoplastic elastomer under the trademark "Santoprene" which has found recent widespread use in the diaphragm industry.
On a macroscopic scale, the thermoplastic elastomer is relatively homogeneous, with the filler being equally blended into the material. The thermoplastic elastomer does not contain fiber strands that wear and fail in response to stress cycles. Although capable of flexing, the "plastic" component of the thermoplastic elastomer typically generates a substantially higher surface hardness than the elastomer (rubber) of the earlier used fabric reinforced elastomeric diaphragms. The harder surface creates undesirable sealing characteristics.
The hardness of the elastomer can be reduced by adding additional rubber to the composition. Unfortunately, the additional rubber also reduces the strength of the diaphragm. It would be desirable to provide a diaphragm that was strong, flexible and provided excellent sealing characteristics.